(1) Field of the Invention
The present invention relates to a focus position adjustment device that adjusts the focus position of a recordable and reproducible optical disc and to an optical disc drive apparatus.
(2) Description of the Prior Art
With the development of multimedia technologies, an optical disc drive apparatus has been widely used as a bulk auxiliary memory in a personal computer or a graphic audio system. In an optical disc drive apparatus, an optical disc such as a compact disc (CD) and a digital video disc (DVD) is used as record medium. Information such as computer data and graphic audio data is recorded on such an optical disc, and the information recorded on the optical disc is reproduced.
In such an optical disc drive apparatus, focus servo and tracking servo have been performed so that information would be recorded or reproduced properly. In this specification, the focus servo refers to control the laser beam directed at an optical disc so that the laser beam would always converge to a certain condition. The focus servo is performed according to a focus error signal that is a signal created from a reflected beam at the laser beam spot by an optical disc. A focus error signal shows a shift of the position of the laser beam on an optical disc (called a "focus position" in this specification). On the other hand, the tracking servo refers to control the laser beam so that the laser beam would follow the track that is formed in a spiral on an optical disc.
An optical disc set in an optical disc drive apparatus is rotated by a spindle motor. When the rotation speed attains a certain level, a laser beam is directed at the optical disc. While the focus servo and the tracking servo are being performed, information is recorded on the optical disc and information recorded on the optical disc is reproduced.
A conventional optical disc drive apparatus in which the focus servo and the tracking servo are performed, however, has problems described below.
First of all, in a conventional optical disc drive apparatus, the focus servo is performed for an optical disc in single spiral land groove format (SS-L/GFMT) without distinguishing a groove track (a concave part of the guide groove of the track that is formed in a spiral) and a land track (a convex part of the guide groove). As a result, it is not always possible to perform the accurate focus servo especially for a high-density optical disc. In this specification, an optical disc in SS-L/GFMT refers to an optical disc on which one turn of recordable and reproducible groove track and land track are alternatively formed from the inner radius to the outer radius and information may be consecutively recorded and reproduced on the land and groove tracks.
When the laser beam reflected by an optical disc of this type passes through an object lens, the diffraction of the reflected beam is influenced by the aberration of the object lens, and the distributions of the diffracted beam for the land track and groove track are different. As a result, the relationships between the zero level of the focus error signal (the control target position of the focus position) and the focus positions for the land track and groove track are different. In other words, in a conventional optical disc drive apparatus, the focus positions are controlled so that a focus position would always converge into the same control target position in the focus servo, and the focus positions for the land tracks and the groove tracks are different. As a result, when the focus positions are adjusted so that the reproduction condition (and the recording condition for a recordable optical disc) would be best for one of the land track and the groove track, it is impossible to obtain the best reproduction condition (and the best recording condition for a recordable optical disc) for the other one.
Secondly, in a conventional disc drive apparatus, it is not considered that the zero level of the focus error signal is not always the best focus position. As a result, the degree of the errors in reproduction is relatively high. In other words, the control target position corresponding to the zero level of the focus error signal and the focus position at which the amplitude of the reproduction signal detected by the optical head is the best (maximum) or the jitter in the reproduction signal is the best (minimum) (called a "best focus position" in this specification) are not always the same. In this case, even when the focus servo is performed according to the control target position, the amplitude of the obtained reproduction signal may be smaller than the original signal and the reproduction jitter may be bigger, so that the number of reproduction errors would be increased. As a result, it is impossible for such a conventional disc drive apparatus to sufficiently record and reproduce data.
Thirdly, for an optical disc in SS-L/GFMT, even when the focus position is adjusted so that the amplitude of the reproduction signal would be maximum or the reproduction jitter is minimum, errors may be produced since the sizes of the amplitude of the reproduction signal and the reproduction jitter are different for the address area and data area. On an optical disc in SS-L/GFMT, address areas are formed between sectors that are areas in which data are recorded. In an address area, the track number and the sector number for identifying a sector are recorded in advance with pits in a different manner in data areas. As a result, adjustment errors are produced in a conventional manner in which the focus position is adjusted using the size of the amplitude of the reproduction signal or the reproduction jitter as the focus position information (the information showing the present focus position, i.e., the shift of the position of the laser beam for the optical disc) since the values shown by the focus position information in the data area and the address area are different.
Fourthly, when the displacement of the face shake that is produced at a certain frequency by the rotation of an optical disc is relatively large, errors are produced for focus position information. As a result, fine adjustment of a focus position is prevented.
Further, for an optical disc in SS-L/GFMT, (1) when a focus position shifts significantly from the best focus position, it is impossible to correctly obtain the address information that is a precondition for the adjustment of the focus position, (2) when an optical disc is shipped, nothing is recorded in the data area on the optical disc. As a result, it is impossible to obtain the focus position information from the data area of an optical disc on which no data is recorded, so that (3) if a test pattern is recorded on an unrecorded optical disc in advance, correct recording can be impossible when the focus position significantly shifts from the best focus position. As a result, when the focus position shifts so significantly that the address information may not be read out, it is impossible to detect the best focus position for an optical disc in SS-L/GFMT
Still further, for a write-protected optical disc in SS-L/GFMT, it is impossible to tentatively record a test pattern on the optical disc when nothing is recorded in the data area.
In addition, for an optical disc in SS-L/GFMT, even when the recording/reproduction processing is correctly performed, the surrounding temperature can change the characteristics of the optical head, and the zero level of the focus error signal that is detected by the optical head, i.e., the control target position in the focus servo can change. As a result, it is impossible to start the detect of the focus position even when the best focus position is detected on the activation of the optical disc drive apparatus and the recording/reproduction processing is correctly performed, when the error rate in the recording/reproduction processing is higher than the acceptable value and the laser beam power in recording reaches the maximum value due to the characteristic changes of the optical head over time.